1. Field of the Invention
The present invention relates to detonators comprising ignition charges and a method for assembling such detonators.
2. Related Art
U.S. Pat. No. 4,727,808, issued Mar. 1, 1988, to Wang et al, discloses an electrically-initiated detonator, an igniting means such as fuse head (9) or an electric resistance wire, low energy detonating cord, NONEL tube or safety fuse (see column 4, lines 41-44 and column 7, lines 21-28) and an initiating charge in initiation relation thereto. The initiating charge comprises a secondary explosive, such as PETN (pentaerythritol tetranitrate), RDX (cyclo-1,3,5-trimethylene-2,4,6-trinitramine), or a mixture thereof, with a particle size that may be below 30 micrometers (.mu.m) and which may be pressed to a density in the range of 1.2 to 1.6 grams per cubic centimeter (g/cc) (see column 5, lines 11-32). The initiating charge is used to initiate the base charge of the detonator. An intermediate charge may be disposed between the initiating charge and the base charge and may have an even lower density, e.g., to 0.8 to 1.4 g/cc (see column 5, lines 33-45). Example 7 shows a test employing PETN at 5 to 15 .mu.m particle size and a tamping of 133 kg (about 8660 psi) for a containment shell having an outer diameter of 6.5 millimeters (mm) and a wall thickness of 0.6 mm.
The "igniting means" mentioned in the Wang et al Patent draw or emit large amounts of energy relative to low energy initiation elements such as SCBs. Further, given the types of igniting means contemplated by Wang et al, the function time for the detonators disclosed therein will be on the order of about 50 microseconds. Because of this prolonged function time, the Wang et al detonators need to provide the confinement and empty chamber in the detonator to prevent the detonator shell from being destroyed by the gaseous products of the ignition charge before the detonation reaction is initiated in the base charge. In the embodiment of FIG. 13, the hollow interior of safety fuse 16 provides the empty chamber for this device.
Fyfe et al, in a paper entitled "BNCP Prototype Detonator Studies Using a Semiconductor Bridge Initiator", discloses the use of BNCP (tetraammine-cis-bis(5-nitro-2H-tetrazorato-N.sup.2) cobalt (III) perchlorate) for use in electric detonators incorporating a semiconductor bridge (SCB) in welded 304 stainless steel confinements. One test device comprised 25 milligrams of BNCP pressed to 10,000 pounds per square inch (psi); another comprised 49 milligrams of BNCP pressed to 20,000 psi. Ignition sensitivity tests for two different particle sizes of BNCP, 15 and 25 microns, performed with a rise time of 15 microseconds, showed that the larger particles took about twice as long to ignite as the smaller particles at 3.5 amps and, at 1.5 amps, the smaller particles ignited but the larger particles did not. In addition, at a fifty-microsecond rise time, the smaller particles were less temperature-sensitive than the larger particles.
The SCB employed by Fyfe et al measured 90.times.270.times.2 .mu.m, and consumed several millijoules of energy to ignite the BNCP. The reported 1 watt, 1 ampere no-fire of these detonators indicates that the BNCP charge was acting like a heat sink that quickly dissipated the ohmic heating of the SCB at the 1 watt, 1 amp no-fire current. Such heat absorption under no-fire conditions indicates that the BNCP was highly compacted.
A manufacturer of BNCP has published product literature suggesting the use of BNCP in place of lead azide as a primary explosive initiating charge and that BNCP is a DDT explosive with a theoretical maximum density of 2.03 g/cc.
U.S. Pat. No. 4,484,960 to Rucker, dated Nov. 27, 1984, discloses a bridgewire detonator comprising a boron/ferric oxide ignition composition. The ferric oxide particles are in the 0.2 to 1.2 .mu.m range. In the example, the ignition composition is loosely loaded into a blasting cap shell in contact with the bridgewire.
Conventional assembly methods for detonators generally comprise sequentially disposing various detonator elements within a metallic detonator shell. Some of the elements comprise pulverulent reactive materials, e.g., the base charge generally comprises a pulverulent secondary explosive, and such materials are typically tamped into the shell at pressures of 10,000 psi or greater. Other components in the detonator may comprise prefabricated units such as a delay unit or an electronic initiation unit which may be pressed into the detonator shell and which may contain compressed pulverulent material therein.